The present invention relates to an electronic copying machine in which a transfer charger, a discharge charger, a discharge lamp and so on are disposed around a cylindrical photosensitive drum.
In general, the various devices for forming an electrostatic latent image are arranged in an electronic copying machine. In particular, a charger, an exposure system, a developing unit, a transfer charger, a separation charger, a discharger, a cleaner, and a discharge lamp are arranged in the direction of rotation of the cylindrical photosensitive drum. In this case, when a copying switch is depressed, the photosensitive drum starts rotating. Simultaneously, the transfer charger, the separation charger and the discharger start operating, and the discharge lamp is lit. DC corona chargers are generally used as the charger and the transfer charger. With the transfer charger, a transfer corona which has a polarity opposite to the polarity of a toner image which is formed on the surface of the photosensitive drum is radiated on the back surface of a copying paper sheet, so that the toner image is transferred to the copying paper sheet. With the separation charger, an AC corona charger is used. A separation corona is radiated to separate the copying paper sheet from the photosensitive drum is such a way that the electric charge on the paper sheet is neutralized. That is, the amount of discharge is large when the AC corona charger has a polarity opposite to that of the transfer charger while the amount of the AC corona charger is small when the AC corona charger has same polarity to that of the transfer charger. The discharge lamp exposes the entire surface of the photosensitive drum which is, in turn, temporarily rendered conductive, so that the electric charge on the surface of the photosensitive drum is neutralized and eliminated. Therefore, in the process for forming an electrostatic latent image beginning from the transfer charger position as shown in FIG. 1, (i) the photosensitive drum is charged by the transfer charger to a predetermined potential; (ii) the surface potential on the photosensitive drum is lowered by a predetermined level when the electric charge which has a polarity opposite to the polarity of the transfer charge is applied by the separation charger and the discharge charger; and (iii) when discharge lamp is exposed the surface potential on the photosensitive drum is lowered to the voltage of around 0 V. The discharger electrically discharges the toner which remains on the surface of the photosensitive drum and generally comprises an AC corona charger. The discharge effectiveness of the AC corona charger in the negative phase is higher than that in the positive phase so that the surface potential of the photosensitive drum is lowered.
When the photosensitive drum is entirely exposed by the discharge lamp, electric charge is grounded which has a polarity opposite to the polarity of a toner image which is formed on the surface of the photosensitive drum. Therefore, when the electric charge is grounded which has a polarity opposite to the polarity of the toner image which is formed on the surface of the photosensitive drum, the surface potential on the surface of the photosensitive drum can be lowered by discharge by the discharge lamp. However, when the electric charge has the same polarity as the polarity of the toner image which is formed on the surface of the photosensitive drum, the surface potential on the photosensitive drum cannot be lowered by discharge by the discharge lamp. Assume that a copying switch is depressed when a prospective area on the photosensitive drum in which a toner image is to be formed in the first copying operation is located between the transfer charger and the discharge charger. The prospective area is first exposed to the separation charger, the discharger and the discharge lamp before reaching the charger at the beginning of the imaging process. This prospective area is not exposed to the transfer charger. As a result, the discharge charger charges the prospective area with electric charge whose polarity is the same as that of the toner image which is to be formed on the surface of the photosensitive drum so that the photosensitive drum can not be discharged by the discharge lamp. Nevertheless, the process for forming the electrostatic latent image on the photosensitive drum is performed after exposure to the discharge lamp. This process is indicated by a broken line as shown in FIG. 1.
When an image is to be applied to an area of the photosensitive drum which lies before the transfer charger with respect to the rotational direction of the photosensitive drum while the drum is at rest, this area is exposed to the transfer charger, the separation charger, the discharger and the discharge lamp before reaching the charger. Thus, this area is charged by the transfer charger with electric charge whose polarity is opposite to that of the toner image which is to be formed on the surface of the photosensitive drum. Then, electric charge whose polarity is opposite to that of the transfer charger and which is less than the electric charge of the transfer charger is stored on the surface of the photosensitive drum by the separation charger and the discharge charger, so that the potential on the surface of the photosensitive drum is lowered by a predetermined level. However, the polarity of the surface potential on the photosensitive drum remains opposite to the polarity of the toner image which is formed on the surface of the photosensitive drum. Therefore the process by the discharge lamp is performed effectively, reducing the charge on the drum to around OV. Then the process for forming an electrostatic latent image on the photosensitive drum is performed, as indicated by a solid line in FIG. 1. Therefore, a potential difference is established between the area of the photosensitive drum which is located between the transfer charger and the separation charger, and the area on the photosensitive drum which is before the transfer charger in the rotational direction of the drum. In other words, an irregular potential distribution occurs on the area which is used for the first copying operation. As a result, a concentration difference occurs within the image of the first copying paper sheet, and between the image of the first copying paper sheet and the subsequent copying paper sheets.